The invention is in the field of manufacturing superconducting magnetic elements. Such elements have a coil of conductors that is operated in the superconductive state.
The manufacture of such superconductive coils requires the accommodation of limitations produced by the physical property differences and the relative variations of the physical properties of the materials employed in the structure when subjected to the conditions encountered by the element in manufacture and operation in service.
Superconducting magnetic elements provide a high field that is achieved by the large current that flows in the conductors of the coil under the essentially zero resistance condition of superconductivity. In the manufacture of such an element, a number of interdependent and sometimes conflicting considerations are encountered.
In the general practice of manufacturing magnetic coil elements the physical property of ductility in the conductor material permits its shaping during winding of the coil and accommodates relative thermal expansion and operating stresses for leads. However, the physical property of ductility cannot be relied on as always being available in the superconductive materials used for conductors.
In general the conductor materials that exhibit superconductivity fall into groups of those considered to be reasonably ductile and those which are considered to be brittle. Materials are now appearing in the art that can only be formed by placing the ingredients in the desired shape and then subjecting that shape to conditions, such as a prolonged heat cycle at high temperature, that completes the reaction of the ingredients. The use of such conductor materials will require extreme care in terminal construction and will add to the temperature range that the element must accommodate.
Each of the materials usable as a superconducting magnetic element conductor has a temperature below which there is an abrupt transition in electrical properties from resistance to superconductive. For conventional Nb based superconducting materials, these transition temperatures can be very low, and the magnetic elements fabricated of these materials are typically operated in the neighborhood of the boiling point of liquid helium, approximately 4.2 degrees K. Thus, the assembly of materials that comprise the magnetic element must be able to withstand relative thermal expansion of the individual parts over a range of temperatures from room temperature where assembly usually occurs, to the high temperature of a required heat treatment to form the superconductor, and to the temperature of superconducting operation, which may be very low.
The performance of a superconductive magnetic element comprising conventional Nb based superconducting material is sensitive to localized inputs of heat. The heat capacities of these materials at the low operating temperatures of these superconductors are very low, and small inputs of heat result in relatively large increases in temperature. When a localized region is heated to the transition temperature and becomes resistive the effect propagates throughout the element producing a dramatic change in electrical performance generally resulting in a malfunction of the element.
One source of localized heat is relative motion between portions of conductors. This situation is usually controlled in the art by impregnation of the coil windings with a material that fills all the interstices between conductors.
Another source of localized heat is relative motion between the conductors of the coil and any housing, such as a coil form member employed in winding the coil and continuing as a support when the element is in service.
Three strategies have evolved in the art to address the localized heat problem. (1) A sufficiently strong bond with the coil form can be provided to prevent the relative motion; (2) a material can be introduced to prevent any bonding to the coil form, and (3) the coil can be removed from the coil form for remounting in a bond free supporting structure. All strategies, however, require consideration of the effect on the terminals of the coil during both manufacture and service.
Magnets constructed of conductors of niobium titanium (NbTi) is an example of prior art ductile superconductors.
Independent of the detailed process employed to fabricate an impregnated niobium titanium coil, the leads have properties characteristic of a ductile alloy and the typical design of leads and terminals reflect these properties. In the prior art, terminals are mounted to a flange or other coil structure, and leads connect the coil and terminals through holes in or around the periphery of the coil form. The leads can incorporate some additional low conductivity material beyond that present generally in the conductor in the windings to aid electrical and mechanical stability. The leads remain ductile to an extent required to accommodate the strains to which they are subjected.
Strategies (1), (2) and (3) above may be adopted with NbTi leads of the prior art. Inherent in strategies (2) and (3) are allowed relative motions of coil form and windings. The strains at the leads associated with the relative motions are accommodated by the ductile nature of the leads. Strategy (1), if successfully applied, eliminates relative motion of coil form and windings, and thereby eliminates significant strains on the leads. However, in the case that strategy is only partially successful and local debonding does occur in the neighborhood of the leads, lead failure is prevented to the extent that the ductile nature of the leads can accommodate the strains.
Magnets constructed of conductors incorporating niobium tin (Nb.sub.3 Sn), including niobium tin with third element additions, is introduced as an example of prior art applicable to brittle superconductors.
Niobium tin coils can be fabricated by one of two methods with respect to the order of winding and reaction of the conductor. In the wind and react technology, the coil is wound of non-reacted conductor. The conductor is then reacted in place on the coil form. In the react and wind technology, the conductor is reacted before winding, usually on a fixture. The coil is then wound of the reacted conductor.
Independent of the detailed process employed to fabricate an impregnated niobium tin coil, the leads have properties characteristic of a brittle compound, and the designs of leads and terminals in the prior art reflect these properties.
In the prior art, terminals are mounted to a flange or other coil structure, and leads connect the coil and terminals through holes in or around the periphery of the coil form. The leads are brittle after the conductor is reacted whether that be before the magnet is wound as in react and wind or after the magnet is wound and heat treated as in wind and react. The strains on the leads must be strictly limited to prevent damage to the superconductors.
Strategy (1) above is usually adopted in the prior art of impregnated Nb.sub.3 Sn coil construction. To the extent that bonding to the coil form eliminates relative motion of coil form and windings at the leads, the Nb.sub.3 Sn leads are protected from detrimental strains. However, in the case that strategy (1) is only partly successful and local debonding does occur in the neighborhood of the leads, lead failure is a possibility due to the brittle nature of the lead material.
Strategy (2) may be adopted within the boundaries of the prior art if a region of strain relief is included in the length of lead between the coil winding and the terminal. A length of lead in the shape of an arc or portion of a loop might be left without rigid support with the intent of accommodating small relative displacements of the constrained ends of the lead with only small strains on the brittle lead itself.
Strategy (3) is outside the prior art to the extent that removal of the coil form from an impregnated Nb.sub.3 Sn coil without special lead support leaves the lead susceptible to damage from even the small strains typical of subsequently connected leads to terminals.